Structure of Lipids

Lipids are a group of hydrophobic molecules that include triglycerides, which are used to store energy, and phospholipids, which are the major structural components of cell membranes.

Triglycerides and most phospholipids consist of glycerol and fatty acid chains.

Fatty acids have methyl groups at one end of the chain and carboxyl groups on the opposite end. The carbon connected to the carboxyl group is called the alpha carbon, and the methyl group carbon is known as the omega carbon.

Fatty acids vary in length and the presence of double bonds in the hydrocarbon chain. Those with double bonds are unsaturated fatty acids and those with only single bonds are saturated fatty acids because they are saturated with the largest number of hydrogen atoms possible.

Fatty acids often have common names but can be systematically named according to the number of carbon atoms and the number and position of the double bonds in the carbon chain.

There are several common numbering methods. The carboxyl referencing system counts the position of all the double bonds from the carboxyl carbon, which is numbered as 1. The omega referencing system counts the position of the double bond closest to the omega carbon, with the omega carbon numbered as 1.

For example, linolenic acid, an omega 3-fatty acid, has 18 carbons and double bonds at positions 9,12, and 15, counting from the carboxyl end. This will be called 18:3 Δ^{9, 12, 15} according to the carboxyl referencing system and 18:3 (ω-3) according to the omega referencing system.

Unsaturated fatty acids can occur in two configurations: cis and trans. In the cis configuration, the hydrogens on the carbons involved in the double bond are on the same side of the bond, whereas in the trans configuration, the hydrogens are located on the opposite sides. 

Structurally, cis fatty acids have bent chains, whereas trans fatty acids have straight chains.

Consumption of trans fats has been found to be responsible for various cardiovascular diseases, whereas consumption of cis fatty acids, such as omega-3 and omega-6, are known to be beneficial to health.

Triglycerides, commonly known as fats, are primarily used for the storage of energy. They are composed of three fatty acids linked to glycerol through ester linkages between the hydroxyl ends of glycerol and the carboxyl ends of the fatty acids resulting in the formation of a nonpolar molecule.

If all the three fatty acids are of the same type, they are known as simple triglycerides. If the three fatty acids differ, they are known as mixed triglycerides.

Another type of lipid, phospholipids, is an important structural feature of biological membranes.

They have hydrophilic heads consisting of phosphate groups modified with a hydrophilic alcohol group and hydrophobic fatty acid tails. The composition of these heads and tails can vary, resulting in different types of membrane lipids.

Glycerophospholipids are a common type of phospholipid that consists of two fatty acids and a highly polar group attached to each carbon of glycerol through ester and phosphodiester linkages, respectively.